The present invention relates to the improvement of filter cartridges made of an accordion fold pack folded in a zigzag-shaped manner that are made of a thermoplastically moldable and heat-sealable material. In filter cartridges of this type, the folds are held at an adjusted, defined distance relative to each other, and are made of a nonwoven fabric. Filter cartridges of this kind are used, for example, in vehicles to filter the passenger compartment air or also as an engine supply air filter. A recurring problem in these filters is the difficulty of providing regular fold spacing and consequently an unvarying, uniformly good filtering action across the entire filter surface. During the installation of the filter or during use, the fold spacings can be changed unintentionally, resulting in a reduced filter action.
The use of prefabricated strips as fixing or positioning means for stabilizing the folds is described in German patent DE OS 42 27 744, in which the strips are joined to the fold edges of the filter material by fusing the strip or the filter sheet material. To build such a filter pack, the filter sheet material folded in a zigzag-shaped manner is inserted into a comb-like holder device, the strips to be applied for stabilizing the fold configuration are put on the fold edges, and the strips are heat-sealed onto the fold edges. In this connection, the placing of the additional strips and heat-sealing the strips to the filter sheet material constitute a disadvantage.
Another way for maintaining the fold spacings in a predefined position can be gathered from German patent DE PS 40 02 078. A filter cartridge designed as an accordion fold pack and made of thermoplastically heat-sealable nonwoven fabric is manufactured with a defined fold spacing. Then, the two outer edges of the accordion fold pack are clamped positively between corresponding, comb-like profiles, and a material laterally projecting over the end edges of the folds is initially folded over and then thermally heat-sealed together. The method is relatively easy to handle and provides good results. Due to the folds heat-sealed at the end edges over their whole height, however, the accordion fold pack gets a stiffness which can restrict its range of application.
A different design approach is pursued in a filter according to German patent DE OS 43 27 368, where supporting elements are inserted into the filter element on one side for fixing the fold spacing in position. The supporting elements are made of flexible plastic and allow the filter element to warp about its longitudinal axis. However, relatively great outlay is required for manufacturing the supporting element and for inserting it into a filter element so that the filter element can be used only conditionally if only for cost reasons.
The object of the present invention is to devise a filter cartridge which can be manufactured in an inexpensive and uncomplicated manner and whose flexibility or stiffness can be predefined. In this context, it is desirable that the spacings of the individual folds of the accordion fold pack always be adjustable and defined. The method required for its manufacture shall be easy to handle and shall not require great outlay for machinery.
The present invention attains these objectives. To manufacture the filter cartridge, in the case of an accordion fold pack folded in a zigzag-shaped manner, the fold peaks are interconnected in a strip-shaped manner in the longitudinal direction of the accordion fold pack transversely to the folds. Within the strip, the individual fold peaks are turned down in the same direction and in each case fixedly (permanently) joined to the next following, likewise turned-down fold at least in the overlapping region. In such a filter cartridge, the distances of the folds relative to each other are defined since all fold peaks are turned down at the same height. Depending on the size of the filter and also as a function of other dimensions of the filter material, the filter surface is provided with only one strip on only one side or also with several strips on only one or both sides of the accordion fold pack. If the accordion fold pack is provided with connecting strips on only one side of the accordion fold pack, the accordion fold pack retains a high flexibility. By strips on both sides of the accordion fold pack, a higher stiffness of the accordion fold pack is achieved.
In principle, it is possible for the turned-down fold peaks to be secured to the next fold in the overlapping region by a bonding process. For this, however, corresponding application means for the bonding compound are required. Therefore, it is more convenient for the filter cartridge to be manufactured from a nonwoven fabric which is composed of a thermoplastically moldable and heat-sealable material. The individual fold peaks can then be joined to the next following fold in the overlapping region by heat-sealing.
In particular in the case of small filters, it is sufficient to attach a strip on only one side of the accordion fold pack, for example, in its central region. In some fields of application of the filter cartridge, this is already sufficient. However, a higher stability is achieved if strips are attached, for example, at each end face of the accordion fold pack, the strips possibly being supplemented additionally by strips in the middle of the accordion fold pack. The attachment in the middle of the accordion fold pack can also be carried out on the opposite side of the accordion fold pack so that the strips are attached on mutually opposing sides. In this manner, the accordion fold pack gets a higher stiffness.
One shape that is favorable for many applications is formed if the strips exist on both sides of the accordion fold pack sack, preferably at its end edges, in a manner that they coincide with one another. Then, a filter cartridge is formed whose longitudinal edges can be used as guide bar, sealing frame, or clamping rim for securing the filter cartridge to a housing.
For manufacturing the filter cartridge, the accordion fold pack is put on a comb-shaped holding device. The teeth of the holding device are shortened by the length of the fold peaks that is capable of being turned-down and mesh with the individual folds. The width of each tooth row is aligned with the width of the respective strip. A heat-sealing tool having one or a plurality of bars, likewise in the width of the individual strips is used for folding over and heat-sealing the fold peaks. In this context, the fold peaks are turned down in the same direction and then heat-sealed together.